Diffuser membrane and method of manufacture

ABSTRACT

A diffuser membrane and a method for manufacturing the same are provided. In the method for manufacturing, a first material is heated and extruded to form a base layer and a second material is heated and extruded to form a coating layer. The base layer and coating layer may be extruded substantially simultaneously in a coextrusion process. Accordingly, the coating may be applied to the base layer in a manner that optimizes the bonding between the two layers and provides the ability to control the thickness of the coating layer. Alternatively, the base layer is formed initially and the coating layer is subsequently formed thereover. The first and second materials have differing properties. The first material may comprise polyurethane and the second material may comprise polyurethane and PTFE.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Divisional of and claims priority to U.S. patentapplication Ser. No. 13/491,718 filed Jun. 8, 2012 to Warrick S. Wadmanet al., entitled “Diffuser Membrane and Method of Manufacture,”currently pending. This Application further claims the benefit of U.S.Provisional Patent Application Ser. No. 61/495,830 filed Jun. 10, 2011to Charles E. Tharp entitled “Diffuser Membrane and Method ofManufacture.” The entire disclosures, including the specifications anddrawings, of all above-referenced applications are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

Flexible membrane diffusers have been used in the diffusion of gasesinto liquids, such as in the aeration of wastewater. The flexiblemembranes have been used with tubular and disc type diffusers. Examplesof each a tubular membrane diffuser and a disc type membrane diffuserare provided in U.S. Pat. No. 7,044,453 to Tharp.

Flexible membrane diffusers are conventionally constructed of rubber ora similar material which is punctured to provide a large number ofperforations. It is known that, in the aeration of wastewater, thehighest efficiency is achieved when the gas is released as fine bubbles.When gas is applied to the diffuser, the gas pressure expands themembrane away from a diffuser body and causes the perforations to openso that gas discharges through them in the form of fine bubbles. Whenthe gas pressure is relieved, the membrane collapses on the diffuserbody to close the perforations and prevent the liquid from entering thediffuser.

Although flexible membrane diffusers are advantageous in many respectsand have achieved widespread acceptance in a variety of gas diffusionapplications, they are not wholly free of problems. In a wastewatertreatment application, materials in the liquid can become deposited onand build up on the membrane to clog or partially clog the perforationsand thus reduce the efficiency of the diffuser. For example, fats,greases and other substances which are commonly found in wastewater canadhere to the membrane. Other substances, including calcium and calciumcompounds, such as calcium carbonate and calcium sulfate, are especiallyproblematic when they precipitate and build up on the diffuser membrane.Biological growth can also build up and compromise the diffuserefficiency. Diffuser membranes can also be chemically degraded bysolvents and various other types of chemicals that may be present in theliquid. This chemical degradation combined with the repeated expansionand contraction of the membrane can weaken the membrane and causepremature structural failure.

One solution to these problems has been to apply a coating to themembrane in order to provide the membrane with a relatively slicksurface that resists biological growth and other materials from beingdeposited thereon. However, the application of the coating is itself notwithout problems. It is often difficult to establish high bond strengthsbetween the membrane's substrate layer and coating layer, in part,because of the non-adhesive qualities of the coating layer. Variousmethods have been proposed to address this problem.

One approach is to use an adhesive, bonding or primer layer between thesubstrate and the coating. By way of example, U.S. Pat. No. 6,759,129 toFukushi discloses the application of a “bonding” layer between thesubstrate and coating and U.S. Pat. No. 7,674,514 to Frankel et al. andU.S. Patent Publication No. 2007/0001323 to Kang disclose theapplication of a “primer” layer between the substrate and coating. Notonly does this approach add additional steps, complications andmaterials in the manufacturing process, but it also results in anincrease of the overall product cost.

Another approach is to apply an uncured film to a pre-cured substrateand curing them together in a mold. By way of example, U.S. Pat. No.7,396,499 to Frankel et al. discloses placing an uncured thinfluoroelastomer film to a pre-cured substrate layer and curing in a hightemperature mold. This approach is also disadvantageous in a number ofrespects. First, because both the substrate and film are in an uncuredstate, it is not possible to optimize both the curing of the substrateand the bonding of the film to the substrate. The time and temperaturerequirements for curing differ from those necessary to achieve optimalbonding between the two materials, so either the curing or the bondingmust necessarily be compromised. The result is a product that has eitheran inadequately cured substrate or an inadequate bond between thelayers. Second, this approach adds additional steps and complications inthe manufacturing process and also results in an increase of the overallproduct cost. Third, there is no opportunity to clean the substratebecause the curing process is interrupted and is only partiallycompleted at the time the film is applied. If contaminants are present,they cannot be removed by solvents or other cleaning processes and caninterfere with the bonding to the point of destroying any ability toproperly bond the materials together. Fourth, the disclosedfluoroelastomer layer must be applied as a film, thus making itimpossible to apply the coating layer in other manners or methods whichmay be preferable in some cases. For similar reasons, the ability tovary the coating thickness is limited. Finally, this may only be used toproduce diffuser membranes that are molded and cannot readily be used toproduce diffuser membranes that are extruded.

There are many shortcomings in these existing configurations and thepresent invention is directed to overcoming one or more, if not all ofthe above shortcomings.

SUMMARY OF THE INVENTION

The present invention involves the provision of a process forconstructing a flexible diffuser membrane for diffusing gas into aliquid. In the process, first and second materials are provided and thenheated to their respective, desired extrusion temperatures. In oneembodiment, the first material comprises polyurethane and the secondmaterial comprises polyurethane and PTFE. The first material is extrudedto form a membrane base layer having first and second surfaces. Thesecond material is extruded to form a membrane film or coating layerover the base layer's first surface. The coating has the ability toresist solvents and other chemicals and to resist adhesion of fat,grease, biological growth and other contaminants in the liquid that candamage conventional elastomers and other membrane materials.

In one embodiment, the base layer and coating are extruded substantiallysimultaneously in a coextrusion process. The two extrusions of the baselayer and the coating layer flow out of the extruder die concentricallyat the same time and are joined immediately upon exiting the die. Inanother embodiment, the base layer is formed initially and the coatinglayer is subsequently formed thereover. In this embodiment, one or moreof the base layer's surfaces may be modified, cleaned and/or have aprimer, adhesive or adhesive catalyst applied thereto in order topromote adhesion between the base layer and the coating. In yet anotherembodiment, a layer of primer or adhesive, located between the baselayer and coating layer, is extruded over the base layer and may beextruded substantially simultaneously with both the base layer andcoating layer.

Upon being formed by the die, the membrane may be cured or cooleddepending upon the type of materials used. The membrane may also beperforated to provide a plurality of perforations extending through themembrane, including the base layer and coating, through which a gas canbe diffused.

The present invention also involves the provision of a coextrudedmulti-layered flexible diffuser membrane. The membrane includes anextruded base layer and an extruded coating disposed over the baselayer. The base layer may comprise polyurethane and the coating maycomprise polyurethane and PTFE. The membrane may be tubular-shaped,disc-shaped, panel-shaped or may be generally flat or of any other shapeor curvature suitable for use as a diffuser. In one embodiment, themembrane includes a base layer having a first coating layer disposed ona first surface and a second coating layer disposed on a second surface.

Other and further objects of the invention, together with the featuresof novelty appurtenant thereto, will appear in the course of thefollowing description.

DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

In the accompanying drawing, which forms a part of the specification andis to be read in conjunction therewith in which like reference numeralsare used to indicate like or similar parts in the various views:

FIG. 1 is a fragmentary perspective view of a portion of an aerationsystem for aerating wastewater that is equipped with a tubular diffuserhaving a flexible membrane constructed in accordance with one embodimentof the present invention;

FIG. 2 is a schematic view of the manufacturing method in accordancewith one embodiment of the present invention;

FIG. 3 is a cross-sectional view of the multiple layer extruded tubulardiffuser membrane shown in FIG. 2 taken generally along line 3-3 in thedirection of the arrows;

FIG. 4 is a cross-sectional view of the multiple layer extruded tubulardiffuser membrane constructed in accordance with another embodiment ofthe present invention; and

FIG. 5 is a perspective view of a multiple layer extruded diffusermembrane in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described with reference to the drawingfigures, in which like reference numerals refer to like partsthroughout. For purposes of clarity in illustrating the characteristicsof the present invention, proportional relationships of the elementshave not necessarily been maintained in the drawing figures.

The following detailed description of the invention references specificembodiments in which the invention can be practiced. The embodiments areintended to describe aspects of the invention in sufficient detail toenable those skilled in the art to practice the invention. Otherembodiments can be utilized and changes can be made without departingfrom the scope of the present invention. The present invention isdefined by the appended claims and the description is, therefore, not tobe taken in a limiting sense and shall not limit the scope ofequivalents to which such claims are entitled.

The present invention is directed to a flexible diffuser membrane and toa process of constructing the membrane. Membranes of this type are usedin various applications in which gases are diffused into liquids. Oneexample is a wastewater treatment system in which flexible membranediffusers are commonly used to diffuse air into the wastewater foraeration and mixing purposes. Flexible membrane diffusers are often usedin this type of application on tubular diffusers, disc diffusers andpanel diffusers.

While FIG. 1 depicts a tubular membrane diffuser generally identified bynumeral 10, it is to be understood that the invention is equallyapplicable to membranes for disc diffusers, panel diffusers and othertypes of diffusers that are used both in water and wastewater treatmentas well as in the diffusion of various types of gases into otherliquids.

The diffuser 10 is used in an aeration system which includes a varietyof air lateral pipes such as pipe 12 which may be floating on thesurface of the liquid or submerged therein. Air or another gas issupplied to the pipe 12 and is discharged into a tee-fitting 14connected with a saddle structure 16 used to mount the diffuser assemblyon the pipe 12. FIG. 1 depicts tubular diffusers 10 extending from eachof the side outlets of the tee-fitting 14 (one shown onlyfragmentarily), although other arrangements are possible. Diffusersystems and structures different from what is shown in FIG. 1 are withinthe scope of the invention.

As shown in FIG. 1, the diffuser 10 includes a hollow rigid diffuserbody 18 which is connected with an outlet of the tee-fitting 14 andextends generally horizontally. The diffuser body 18 is provided withone or more openings (not shown) which discharge the gas within aflexible membrane 20 secured to the diffuser body 18 by band clamps 22or other suitable fasteners. The membrane 20 is provided with aplurality of small perforations 24 which may take the form of slitsarranged in any desired pattern.

When air is applied to the diffuser body 18 from the lateral pipe 12,the gas pressure causes the membrane 20 to expand from the diffuser body18, thus opening the perforations 24 and discharging the gas through theperforations into the liquid in the form of fine bubbles which arebeneficial in that they efficiently transfer the gas to the liquid. Whenthe gas pressure is relieved, the flexible membrane 20 collapses backonto the diffuser body 18 and thus closes the perforations 24 so thatthe liquid is unable to leak into the diffuser.

The present invention is directed specifically toward a multi-layerdiffuser membrane 20 comprising at least two types of materials andbeing formed in an extrusion or coextrusion process. One embodiment ofthe present invention includes a two-layer membrane including astructural layer on the bottom or innermost layer and film or coatinglayer on the top or outer-most layer. Another embodiment includes athree-layer membrane having a structural layer, a first coating layerdisposed on a first surface of the structural layer and a second coatinglayer disposed on a second surface of the structural layer.

FIG. 2 illustrates a schematic drawing of a process for manufacturing anextruded diffuser membrane 20. In the process, the raw material for thebase layer 26 can be contained within a hopper 38 or other storagereceptacle. From there, the base material may proceed into a mixer 40wherein it is mixed to provide a homogeneous mixture. The base materialis then transferred through a feed tube 42 to an extrusion element,mandrel or die 44. The feed tube 42 may contain a screw-feed mechanismand may include heated walls to melt the raw base material to a desiredextrusion temperature. Alternatively, heat may be applied in the mixer40 thus creating a flowable homogeneous base mixture prior to being fedto the die 44 through the feed tube 42.

Similarly, the raw material for the film or coating layer 28 can becontained within a hopper 46 or other storage receptacle. From there,the coating material may proceed into a mixer 48 wherein it is mixed toprovide a homogeneous mixture. The coating material is then transferredthrough a feed tube 50 to a die 44. The feed tube 50 may contain ascrew-feed mechanism and may include heated walls to melt the rawcoating material to a desired extrusion temperature. Alternatively, heatmay be applied in the mixer 48 thus creating a flowable homogeneouscoating mixture prior to being fed to the die 44 through the feed tube50.

In one embodiment, the base layer 26 and coating layer 28 are formedsubstantially simultaneously with one another in a coextrusion process.The membrane 20 may be formed as an integral, one-piece component in asingle operation. Accordingly, the coating 28 may be applied to the baselayer 26 in a manner that optimizes the bonding between the two layersand provides the ability to control the thickness of the coating layer28.

In another embodiment, the base layer 26 is formed initially and thecoating layer 28 is subsequently formed thereover, albeit not in asimultaneous coextrusion process. In this embodiment, one or more of thebase layer's surfaces 30 and 32 may be modified, cleaned and/or have aprimer, adhesive or adhesive catalyst applied thereto, either an in-lineor off-line process, in order to promote adhesion between the base layer26 and the coating layers 28 and/or 36. The base layer 26 may then beconveyed by a conveying mechanism in order for the coating 28 to beextruded thereon.

In yet a another embodiment, a layer of primer or adhesive locatedbetween the base layer 26 and coating layer 28 is extruded over the baselayer 26 and may be extruded substantially simultaneously with both thebase layer 26 and coating layer 28.

The extruder die(s) 44 may be configured to create a tubular diffusermembrane 20, a generally flat diffuser membrane 20 a or a membrane ofany other suitable shape. The dies(s) 44 may be configured to create abase layer 26 or coating layer 28 that is relatively thick or thin asdesired or specified. The ability to control the thickness of thecoating layer 28 provides the ability to specifically manufacture adiffuser membrane 20 having a coating layer 28 that is thick enough towithstand erosion thereof.

Upon being found by the die, the membrane 20 or 20 a may be cured orcooled depending upon the type of materials used. The membrane 20 or 20a may also be perforated to provide a plurality of perforations 24extending through the membrane 20 or 20 a, including the base layer 26or 26 a and coating layers 28, 28 a and 36, through which a gas can bediffused.

FIG. 3 depicts a two-layered membrane 20 resulting from the processdescribed above. This particular embodiment has a tubular cross sectionand includes an inner base layer 26 and an outer coating layer 28. Thebase layer 26 has an outer or first surface 30 and an inner or secondsurface 32. As shown, the coating 28 is disposed over the base layer'sfirst surface 30. The base layer 26 and coating 28 may be each be of anysuitable thickness.

FIG. 4 illustrates a three-layered membrane 20 having a tubularcross-section that includes a base layer 26, an outer coating layer 28and an inner coating layer 36. The outer coating 28 is disposed over thebase layer's first surface 30 and the inner coating 36 is provided tothe interior of the base layer's second surface 32. In such anembodiment, the number of hoppers, mixers and feed tubes shown in FIG. 2can be increased in order to accommodate the third layer. Further, itwill be understood that embodiments having four or more layers are alsowithin the scope of this invention.

FIG. 5 shows a generally flat membrane 20 a as may be associated with adisc diffuser or a panel diffuser. The two-layered membrane 20 a isshown as having a base layer 26 and a coating layer 28 a. The base layer26 a has an outer or first surface 30 a and an inner or second surface32 a. As shown, the coating 28 a is disposed over the base layer's firstsurface 30 a. It will be understood that a second coating (not shown)may be extruded over the base layer's second surface 32 a.

In any of the embodiments that are within the scope of the presentinvention, the material forming the base layer 26 and 26 a may compriserubber, polyurethane, ethylene propylene diene monomer (EPDM), nitrilerubber, nitrile butadiene rubber, thermoset material, thermoplasticmaterial, thermoplastic vulcanizate (TPV), thermoplastic rubber (TPR),natural or artificial woven material, polyvinyl chloride (PVC),polytetrafluoroethylene (PTFE), any product from the urethane-basedfamily or urethane family of thermoplastics, a cross link polyurethanematerial, any polymer suitable for bonding with polyurethane or PTFE,any other suitable material now known or hereafter developed or anycombination thereof.

The material forming the coating layer 28, 28 a and 36 may comprisepolyurethane, PTFE, silicone, fluorinated ethylene propylene,fluorocarbon elastomer, fluoroelastomer polymer, ceramic, thermoplastic,thermoset material, a suitable fluorine-containing material, a crosslink polyurethane material any polymer that bonds with polyurethane orPTFE, any other suitable non-stick or low friction material, any othersuitable material now known or hereafter developed or any combinationthereof. One embodiment includes a mixture of polyurethane and PTFEwherein polyurethane comprises a range of 1%-99% of the total mixture byweight and the PTFE comprises 99% to 1% of the total mixture by weight.

An adhesive catalyst may also be applied to the mixture making up thebase layer 26 or 26 a and/or coating layers 28, 28 a and 36 for creatinga strong adhesive, chemical and molecular bond. The adhesive catalystcan be applied in order to increase the strength of the bond between thebase layer 26 or 26 a and coating layer(s) 28, 28 a and 36 such that thebond is able to withstand the forces applied to the membrane 20 and 20 aduring normal service.

The coating layers 28, 28 a and 36 (if provided) prevent contaminants inthe liquid from becoming deposited on and accumulating on the membrane20 or 20 a, as the coating layers 28, 28 a and 36 provide a slick ornonstick surface that resists adhesion of foreign materials to an outersurface 34 or 34 a of the membrane 20 or 20 a. The coatings 28, 28 a and36 are also beneficial in that they resist the growth of biologicalmaterials that could otherwise build up on the membrane 20 or 20 a. Thecoatings 28, 28 a and 36 are also resistant to chemicals and othersolvents that can chemically attack and degrade or destroy the membrane28, 28 a and 36.

Accordingly, the membrane 20 and 20 a of the present invention takesadvantage of the beneficial attributes of the base layer 26 or 26 a(physical and structure properties and flexibility) along with theprotective qualities provided by the coating layers 26, 26 a and 36.

One embodiment of the present invention includes a coating layer 28comprising polyurethane and PTFE that is extruded substantiallycontinuously and simultaneously with a base layer 26 comprisingpolyurethane. The polyurethane of the base layer 26 bonds or adhereswith the polyurethane of the coating layer 28 to form a polymer bondbetween the two layers. In such a case, no adhesive or independentmaterials need be applied between the two layers 26 and 28 in order forthe two layers to form an adequate bond. In the process of constructingthis embodiment of the membrane 20, PTFE is mixed with polyurethane toform the material from which the coating layer 28 is extruded. The twoextrusions of the base layer 26 and the coating layer 28 flow out of theextruder die 44 concentrically at the same time and are joinedimmediately upon exiting the die 44. The layers 26 and 28 are cooled,cured and bonded together permanently during this manufacturing process.

From the foregoing, it will be seen that this invention is one welladapted to attain all the ends and objects hereinabove set forthtogether with other advantages which are obvious and which are inherentto the structure. It will be understood that certain features and subcombinations are of utility and may be employed without reference toother features and sub combinations. This is contemplated by and iswithin the scope of the claims. Since many possible embodiments of theinvention may be made without departing from the scope thereof, it isalso to be understood that all matters herein set forth or shown in theaccompanying drawings are to be interpreted as illustrative and notlimiting.

The constructions described above and illustrated in the drawings arepresented by way of example only and are not intended to limit theconcepts and principles of the present invention. Thus, there has beenshown and described several embodiments of a novel invention. As isevident from the foregoing description, certain aspects of the presentinvention are not limited by the particular details of the examplesillustrated herein, and it is therefore contemplated that othermodifications and applications, or equivalents thereof, will occur tothose skilled in the art. The terms “having” and “including” and similarterms as used in the foregoing specification are used in the sense of“optional” or “may include” and not as “required”. Many changes,modifications, variations and other uses and applications of the presentconstruction will, however, become apparent to those skilled in the artafter considering the specification and the accompanying drawings. Allsuch changes, modifications, variations and other uses and applicationswhich do not depart from the spirit and scope of the invention aredeemed to be covered by the invention which is limited only by theclaims which follow.

1-9. (canceled)
 10. A coextruded multi-layered flexible diffusermembrane structure for applying gas to a liquid comprising: an extrudedbase layer formed from a first material, said base layer having firstand second opposing surfaces; an extruded first coating layer formedfrom a second material, said first coating layer being disposed on saidfirst surface of said base layer and being simultaneously coextrudedtherewith; and a plurality of perforations extending through said baselayer and coating layer through which gas can be diffused into saidliquid, wherein said perforations are slits.
 11. The diffuser membraneof claim 10 further comprising an extruded second coating layer formedfrom a third material, said second coating layer being disposed on saidsecond surface of said base layer.
 12. The diffuser membrane of claim11, wherein said membrane is selected from a group consisting of atubular-shaped membrane, a disc-shaped membrane and a panel-shapedmembrane.
 13. The diffuser membrane of claim 10, wherein said slits arearranged in a pattern.
 14. The diffuser membrane of claim 10, whereinsaid first material comprises at least one of rubber, polyurethane,EPDM, thermoplastic vulcanizate, PVC, and PTFE.
 15. The diffusermembrane of claim 14, wherein said second material comprises at leastone of polyurethane, PTFE, silicone, and fluorinated ethylene propylene.16. The diffuser membrane of claim 10, wherein said first materialcomprises polyurethane and said second material comprisespolytetrafluoroethylene.
 17. The diffuser membrane of claim 10, whereinat least one of said first material and said second material comprisesan adhesive catalyst.
 18. The diffuser membrane of claim 10, wherein apolymer bond is formed between said base layer and said first coatinglayer.
 19. A coextruded multi-layered flexible diffuser membrane forapplying gas to a liquid constructed by a process comprising the stepsof: extruding a base layer from a first material, said base layerincluding first and second opposing surfaces; extruding a first coatinglayer from a second material over said first surface of said base layer;and perforating said membrane to form a plurality of perforationsextending though said membrane, including extending through said baselayer and said first coating layer through which said gas can bediffused into said liquid; wherein said perforations are slits; whereinsaid base layer and said first coating layer are extruded simultaneouslyin a coextrusion process.
 20. The diffuser membrane of claim 19, whereinsaid slits are arranged in a pattern.